As for the primary structure of STF, Jean-Fransois Bach et al. reported in 1977 that an active ingredient extracted from swine serum showing activities as STF had the following structure: EQU pGlu-Ala-Lys-Ser-Gln-Gly-Gly-Ser-Asn-OH.
[refer to J. Biol. Chem., 252 8045(1977)].
The thymus is an organ having close relationships to antibody-production and cellular immunity, and it has been known that several peptides considered to be secreted from the thymus have actions of regulating the immunological functions. The action of STF on the thymic cells to increase the thymic cyclic-AMP resulting in contribution to the differentiation of thymic cells has been indicated by A. Astaldi et al. [refer to Nature (London), 260, 713(1977)], and thus STF is an interesting peptide as a factor secreted from the thymus and contained in the serum.
In the case where the immuno-regulating action of STF is utilized clinically or in other purposes, it is necessary to accurately catch the concentration of STF in blood, and that the concentration of STF in blood is in the order of several pg/ml has been reported by Jean-fransois Bach et al. (refer to Bullein de L'Insututut Pasteur, 1978, 325). Although it is effective to carry out radioimmunoassary (RIA) in analyzing such a substance of presence in a minute amount, as is seen its amino acid-composition, STF does not contain such an amino acid having aromatic ring(s) which is possibly labelled with radioactive iodine as in histidine. Accordingly, in order to carry out RIA, it is necessary to introduce a structure easily subjected to radio-labelling into the substance in advance of using such a substance to RIA. Moreover, although a plenty of pure STF and radio-labelled STF is necessary to carry out RIA of STF, it is not a profitable method to collect a plenty of STF from animal blood and in a pure form when the amount of presence of STF present in nature, the difficulty of purfying STF, etc. are considered.
Accordingly, the synthesis of easily radio-labellable derivatives of STF has been expected.